The MkII contains two reverbs. The first is a Modern Reverb. The second is a Spring Reverb based on the one contained within the original MkII.

This determines which keyboard manual the Reverb Effect is assigned to.
Switches between the Modern Reverb (I) and Spring Reverb (II)
When using the Modern Reverb this knob allows you to select from small, through to large Reverb space sizes.
When using the Spring Reverb this you to select from small, through to large spring lengths.
Adjusts the Decay time of the selected Reverb.
Adjust the level or gain of the Reverb signal.
